In computer networks such as the internet, computers that are run by malicious programs and that automatically access servers pose a problem. Such computers automatically execute processes presumed to be originally human-operated, such as automatically acquiring or logging into various accounts provided by a server.
A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is used for the server to determine whether or not the user terminal is being operated by a human. A CAPTCHA is a test which a server makes a user terminal execute, and is easy for a human to answer correctly but difficult for a program to answer correctly. Based on the answer from the user terminal, the server determines whether the answerer is a user terminal operator or a program running in the user terminal.
An image is displayed on a display screen of the user terminal and the image is used as the CAPTCHA. Known examples of CAPTCHAs that use an image include those disclosed in Patent References 1 to 3. Patent Reference 1 discloses a method in which problem text is displayed as an image and a user inputs an answer to the problem text by manual input. Patent Reference 2 discloses a method of displaying a subject image and a processed image which is a predetermined image process added to the subject image, and simultaneously displaying problem text relating to these images. Patent Reference 3 discloses a method of displaying an animation and having the user input text partially shown within the animation.